Field of the Invention
The present invention relates to a dielectric-thin-film forming composition, which can form a thin-film capacitor or the like having excellent leak current characteristics, high tunability, and a high dielectric constant; a method of forming a dielectric thin film; and a dielectric thin film formed by the method. In this specification, the term “tunable” means that when an applied voltage is changed, the capacitance is changed. The term “tunability” means the variability or the rate of change of the capacitance.
Description of Related Art
In a high-frequency tunable device such as a high-frequency filter, a high-frequency antenna or a phase shifter or the like, a variable capacity element (tunable element) including an upper electrode, a lower electrode and a thin-film capacitor composed of a dielectric layer formed between the electrodes, is installed. The thin-film capacitor functions as a capacitor in which the capacitance is changed by a change in a voltage which is applied between the electrodes. As the dielectric layer of the thin-film capacitor as described above, a dielectric thin film formed by using a perovskite-type oxide such as strontium titanate (SrTiO3), barium strontium titanate (hereinafter, referred to as “BST”), barium titanate (BaTiO3) or the like, having a high dielectric constant has been used. As a method of forming a dielectric thin film, a chemical solution method such as a sol-gel method has been used in addition to a physical vapor-phase growth method such as a vacuum deposition method, a sputtering method, or a laser ablation method or a chemical vapor-phase growth method such as a chemical vapor deposition (CVD) method (for example, see Japanese Unexamined Patent Application First Publication No. S 60-236404).
The thin-film capacitor installed in the high-frequency tunable device is required to have variability (tunability) in its capacitance with respect to an applied voltage. Accordingly, it is desirable that the range of variation of the capacitance that can be controlled when the voltage is applied is increased, that is, it is desirable for the thin-film capacitor to have high tunability. The reason is that as the tunability increases, a wider resonant frequency band can be handled with a smaller voltage change. In greater detail, the tunability is expressed by the formula (C0v−Ctv)/C0v×100% where the capacitance before the application of a voltage is denoted by C0v and the capacitance after the application of a voltage of t V is denoted by Ctv. For example, as shown in FIG. 1, when a voltage of 5 V is applied, the capacitance is changed from C0v, when no voltages are applied to C5V. At this time, it can be said that the greater the variation from C0V to C5V is, the higher the tunability, and that a thin-film capacitor having high tunability is achieved. As a technique for increasing the tunability, a tunable capacitor has been disclosed which maintains a desired impedance when used in a high-frequency band and can secure high tunability by using a material having a high dielectric constant (for example, see Japanese Unexamined Patent Application First Publication No. 2008-53563).